Since a silica aerogel, as a high specific area, ultra-porous material having a porosity of about 90% to 99.9% and a pore diameter of about 1 nm to 100 nm, has excellent characteristics such as ultra lightweightness, ultra insulation, and ultra-low dielectric constant, research into the applications of the aerogel as a transparent insulator and an environmentally-friendly high-temperature insulator, an ultra-low dielectric thin film for a highly integrated device, a catalyst and a catalyst support, an electrode for a supercapacitor, and an electrode material for desalination as well as the development of an aerogel material has been actively conducted.
The biggest advantage of the silica aerogel is super-insulation having a thermal conductivity of 0.300 W/m·K or less which is lower than that of a typical organic insulation material such as a Styrofoam. Also, the aerogel may address fire vulnerability and generation of toxic gas in case of fire, i.e., fatal weaknesses of a typical organic insulation material.
In order to prevent structural collapse due to a shrinkage phenomenon occurred during drying, the silica aerogel is prepared by a method in which a hydrophobic silica aerogel is prepared and a surface modifier is then removed by pyrolysis.
Specifically, the silica aerogel is prepared by the steps of: preparing a silica sol by hydrolysis of tetra ethyl ortho silicate (TEOS) or water glass with an acid catalyst, adding a basic catalyst thereto, and performing a condensation reaction to prepare a hydrophilic wet gel (first step); aging the wet gel (second step); performing solvent substitution in which the aged wet gel is put in an organic solvent to substitute water present in the wet gel with an organic solvent (third step); preparing a hydrophobic wet gel by adding a surface modifier to the solvent-substituted wet gel and performing a modification reaction for a long period of time (fourth step); preparing a hydrophobic silica aerogel by washing and drying the hydrophobic wet gel (fifth step); and pyrolyzing the aerogel (sixth step).
Recently, in order to further extend applications of silica aerogel, a plan of improving mechanical properties in addition to original properties of the silica aerogel has been reviewed, and, for example, a metal oxide-silica composite aerogel, in which a metal oxide is introduced, is being developed.
In general, the metal oxide-silica composite aerogel is being prepared by the steps of: adding a metal ion solution and an acid catalyst to a water glass solution and performing a reaction to prepare a metal oxide-silica composite wet gel (step 1); and washing and oven drying the wet gel (step 2) (see FIG. 1). However, with respect to the metal oxide-silica composite aerogel prepared by the above preparation method using the oven drying, since the drying slowly proceeds from a surface of the metal oxide-silica composite wet gel by heat conduction during the oven drying, heat transfer to the inside of the wet gel is delayed, and thus, the drying may non-uniformly occur. Accordingly, overall drying time is increased to generate a severe pore shrinkage phenomenon due to surface tension of a solvent in the wet gel, and, as a result, since a specific surface area and a pore volume of the prepared metal oxide-silica composite aerogel are significantly reduced, the metal oxide-silica composite aerogel may have physical properties unsuitable for industrial applications. Also, the step of washing the wet gel with an organic solvent having a low surface tension before the drying is performed to suppress the shrinkage phenomenon, but, since a shrinkage phenomenon suppression effect is limited, it is not suitable for the preparation of a metal oxide-silica composite aerogel having high specific surface area and high pore volume and economic efficiency may be reduced because a large amount of the organic solvent is required.
Thus, there is a need to develop a method which may prepare a metal oxide-silica composite aerogel having high specific surface area and high pore volume characteristics due to the fact that the shrinkage phenomenon during drying is effectively suppressed, while having good economic efficiency because the large amount of the organic solvent is not required.